1. Field of the Invention
The present invention relates to improvement in the process for cleaning a silicon mass such as a polycrystalline silicon mass with an acid. More particularly, the present invention relates to a process for cleaning a silicon mass in which nitric acid is effectively recovered for reuse from the waste solution of a treatment solution which is used as a cleaning solution or an etchant and comprised mainly of a mixed acid composed of nitric acid and hydrofluoric acid, as well as to a process for removing such an acid adhering to a silicon mass, more specifically a process for cleaning a silicon mass, particularly a polycrystalline silicon mass, in which the silicon mass is etched with such an acid and then the acid adhering to the silicon mass is removed rapidly.
2. Description of the Prior Art
A treatment solution comprised mainly of fluoro-nitric acid, which is a mixture of nitric acid and hydrofluoric acid has heretofore been used as a cleaning liquid or an etchant for a polycrystalline silicon mass or a silicon wafer. In the conventional acid-washing processes, the treatment solution which contains impurities at high concentrations and is thus no more usable is disposed of as a waste. However, the waste treatment solution still contains nitric acid in an amount of 40 to 50% by weight and is a high concentration, strongly acidic solution, and hence the waste disposal must be subjected to a neutralization treatment or the like which incurs unnegligible cost. The waste disposal raises a more severe problem as the amount of silicon to be treated increases. Therefore, it is desired to recover nitric acid and the like from the waste treatment solution to decrease cost incurred in the waste disposal and reuse nitric acid.
Despite attempts to recover nitric acid from the waste solution produced and reuse it in the process of cleaning silicon, it has been difficult to recover nitric acid effectively. For example, for the surface treatment of stainless steel plates, there have been known several processes for recovering acids from a waste acid solution containing a mixed acid composed of nitric acid and hydrofluoric acid by distillation (Japanese Patent Publication No. 11755/1981, etc.). In actuality, when attempts were made to separate the waste acid by distillation according to the conventional processes, the piping would be clogged to make it difficult to continue the distillation so that the separation and recovery of nitric acid, etc., are difficult in the case where the waste solution from the silicon treatment process is handled.
In addition to the problem of disposal of the waste cleaning solution, the acid-washing of a silicon mass has a problem in rinsing after the acid-washing as described below. For example, in the production of a highly pure polycrystalline silicon, the cleaning is carried out after the silicon is grown in the form of a rod, and cut or crushed to an appropriate size to form a cylindrical mass (referred to as "chunk") or a small mass (referred to as "lump"). As already stated, the cleaning is performed by etching the silicon mass with an etchant comprised mainly of hydrofluoric acid and nitric acid to remove impurities adhering to and an oxide coating formed on the surface of the silicon mass, and then rinsing the remaining etchant with purified water. Thereafter, the cleaned silicon mass is transferred to a drying step.
Since the etchant is a strongly acidic solution, it is necessary to remove the solution as completely as possible in order to obtain a clean silicon mass. However, the silicon mass obtained as described above has a complicated surface structure having minute hollow portions as viewed on a microscopic scale so that the acid used as the etchant spreads in the hollow portions. As a result, mere spraying of water onto the surface of the silicon mass is insufficient for the acid which has permeated or adhered to the surface of the silicon mass to be removed completely. For this reason, the silicon mass is immersed in a purified water bath to remove the acid as completely as possible.
Although this process is advantageous in that it is simple, it takes a long time in the cleaning step. At present, the silicon mass must be immersed for one night while exchanging rinsing water several times to decrease the amount of nitric acid ion which remains after the cleaning treatment to an acceptable level for practical use. In the immersion cleaning process, not only a large space is required for an immersion bath but also production of a large amount of purified water and recovery thereof are necessary. Furthermore, in the process, the acid is removed by spontaneous diffusion, and hence it is often the case that the removal of the acid is incomplete depending on the condition of the surface of the silicon mass. Therefore, there has been a keen desire to rationalize the process by enabling the removal of acid with a smaller amount of purified water and in a shorter time.